1. Field of the Invention
The present invention relates to an optical transmission and receiving module usable for an optical communication system for transmitting and receiving optical signals through an optical fiber, and specifically to an optical transmission and receiving module for realizing high speed transfer based on, for example, IEEE1394 and USB2.
2. Description of the Related Art
Japanese Laid-Open Publication No. 7-248429 discloses an optical transmission and receiving module 1000 as shown in FIG. 30. The optical transmission and receiving module 1000 operates in the following manner.
Transmission signal light which is emitted by a light emitting element 1001 is transmitted through a cover glass 1060 attached to a package and branched into two light components by a Foucault prism 1003. The light components are collected to point A and point B by a lens 1004. That is, only one of the light components is incident on an optical fiber 1007.
Receiving signal light which is output by the optical fiber 1007 is collected by the lens 1004 and then incident on the Foucault prism 1003. The light is branched into two light components by the Foucault prism 1003 and then transmitted through the cover glass 1060. One of the two light components is incident on a light receiving element 1002.
Japanese Laid-Open Publication No. 8-15582 discloses another optical transmission and receiving module 2000 as shown in FIG. 31. The optical transmission and receiving module 2000 operates in the following manner.
Transmission signal light which is emitted by a semiconductor laser 2002 is collimated by a lens 2004. The light is then incident on a holographic diffraction grating 2005 to be branched into a zeroth-order light component and a first-order light component. Only the zeroth-order light component, which is collected, is incident on an optical fiber 2006.
Receiving signal light which is output by the optical fiber 2006 is incident on the holographic diffraction grating 2005 and branched into a zeroth-order light component and a first-order light component. Both light components are collimated. Only the first-order light component, which is collected by the lens 2004, is incident on a light receiving element 2003.
The optical transmission and receiving module 1000 shown in FIG. 30 has the following problems.
(1) Since the vertex angle of the Foucault prism 1003 is as small as 2.degree. to 3.degree., the light emitting element 1001 and the light receiving element 1002 are inevitably located close to each other. Accordingly, the light receiving element 1002 needs to be located far from a focal point 1008 of the receiving signal light. Therefore, the receiving signal light needs to be detected in an expanded state. This requires the light receiving element 1002 to be larger. Such a large light receiving element 1002 has a capacitance which is too large to perform high speed transmission.
Although it is conceivable to increase the vertex angle of the Foucault prism 1003 in order to extend the distance between the light emitting element 1001 and the light receiving element 1002, such an arrangement requires the Foucault prism 1003 to be thicker. This makes difficult the size reduction of the optical transmission and receiving module 1000.
(2) The Foucault prism 1003 needs to be located at a high precision since when the Foucault prism 1003 is not on an optical axis of the optical fiber 1007, the branching ratio of the Foucault prism 1003 is changed from the designed ratio.
(3) When an RCLED (resonant cavity light emission diode) is used for the light emitting element 1001, the light utilization factor is reduced since the peak radiation angle of the light generated at the high efficiency RCLED is not 0.degree..
The optical transmission and receiving module 2000 shown in FIG. 31 has the following problems.
(1) The wavelength of the light changes in accordance with the temperature of the semiconductor laser 2002. When the wavelength of the light incident on the holographic diffraction grating 2005 changes, the collection position of the light and the diameter of the light spot incident on the light receiving element 2003 change. Thus, the light receiving sensitivity of the optical transmission and receiving module 2000 is reduced.
(2) Since the diffracted light from the holographic diffraction grating 2005 includes a high-order diffracted light component, which is unusable for communication, the light utilization factor of the optical transmission and receiving module 2000 is reduced.
(3) Production of a sawtooth-shaped diffraction grating, which is required to suppress the high-order diffracted light component, is difficult due to the microscopic pitch of the grating.
(4) When an RCLED is used for the light emitting element instead of the semiconductor laser 2002, the light utilization factor is reduced since the peak radiation angle of the light generated at the high efficiency RCLED is not 0.degree..